1. Field of the Invention
The present invention relates to an ink jet printing device including a print head formed with a plurality of nozzles for selectively ejecting ink droplets onto a recording medium based on print data so as to form images on the recording medium, and also to an ink supply method for supplying the print head with ink.
2. Related Art
There has been provided a conventional ink jet print device that performs an image forming operation for forming images on a recording medium. Main components of such a conventional ink jet print device are shown in FIG. 8. As shown in FIG. 8, the ink jet print device includes a print head 301 formed with several hundreds of nozzles 302 (only one is shown in FIG. 8), a common ink chamber 307, a supply path 308, and a sub ink tank 309. A motor (not shown) is provided for reciprocally moving the print head 301 in a direction that is perpendicular to the sheet surface of FIG. 8. A recording medium 313 is placed in confrontation with the nozzles 302.
Each nozzle 302 includes an orifice 303, an ink chamber 305, and a restrictor 306, all are in fluid communication with one another. The restrictor 306 regulates the amount of ink that is supplied from the common ink chamber 307 into the ink chamber 305. A recording medium 313 is placed in confrontation with the orifices 314. A piezoelectric element 304 is mounted on a partition wall of the ink chamber 305. The piezoelectric element 304 is an energy generating member that is deformed and undeforms based on driving signals.
The sub ink tank 309 is connected to a pressure pump 311 via a deformable duct 312, and further to a main ink tank 310.
With this configuration, the image forming operation is performed by selectively ejecting an ink droplet 314 through the orifices onto the recording medium 313 while reciprocally moving the print head 301. The ink droplet 314 is ejected through the orifice 303 in the following manner.
That is, first, the piezoelectric element 304 is deformed based on a driving signal, so that the volume of the ink channel 305 increases. As a result, internal pressure of the ink channel 305 decreases, and ink in the common ink chamber 307 is introduced into the ink chamber 305 through the ristrisctor 306. Next, the deformation of the piezoelectric element 304 is released. The volume of the ink chamber 305 drops to its initial amount, so the internal pressure of the ink chamber 305 increases. As a result, an ink droplet 314 is ejected through the orifice 303. Each time an ink droplet 314 is ejected, ink in the sub ink tank 306 is introduced into the common ink chamber 307 through the supply path 308 to supplement the consumed ink.
In the above-described print head, ink level in the ink tank 309 is set lower than the position of the orifice 303 by a level difference Ho in a vertical direction. In this way, ink in the ink chamber 305 is prevented from leaking through the orifice 303. Also, a sensor 315 is provided to the sub ink tank 309 for detecting the remaining amount of ink in the sub ink tank 309. As the ink is consumed, the ink level is lowered and the level difference Ho increases. When the sensor 315 detects that the level difference Ho becomes greater than a predetermined height, then the pressure pump 311 supplies ink from the main ink tank 310 into the sub ink tank 309. That is, the pressure pump 311 sucks up the ink from the main ink tank 310, and then applies pressure to the sucked-ink. As a result, a predetermined amount of ink is supplied into the sub ink tank 309 via the duct 312.
Also, the ink in the sub ink tank 309 is exposed to the ambient air through openings 399. Because the sub ink tank 309 is not sealed off from the outside, pressure applied to the nozzle 302 will not greatly fluctuate even when the print head 301 is reciprocally moved. Also, because the main ink tank 310, which has relatively a large volume, is not mounted on the print head 301, the motor for driving the print head 301 can be smaller.
However, in the above-described ink jet print device, when the piezoelectric elements increase and decrease the internal pressure of the ink chamber 305 for ejecting the ink droplet 314, air bubbles are generated in the ink in the nozzles 302. Such air bubbles prevent proper ink ejection, so degrade image quality.
Because the print head 301 is formed with a large number of nozzles 302, image forming can be performed at high speed. However, when the print head 301 is formed with a larger number of nozzles 302, air bubbles are more likely to be generated.
Also, when the print head 301 is reciprocally moved, the velocity of the print head 301 is repeatedly accelerated and decelerated. This acceleration and deceleration changes pressure applied to the ink in the print head 301, especially when the moving direction of the print head 301 is reversed. As a result, air bubbles are easily generated.
Moreover, air bubbles are also generated in the main ink tank 310 and the duct 312. When these air bubbles are supplied to the print head 301 along with ink, the air bubbles also prevent proper ink ejection.
In order to overcome the above-described problems, the ink jet print device executes a purging operation for forcefully removing such air bubbles. During the purging operation, first, the print head 301 is moved to a predetermined purging position that is outside of a recording region. A purging mechanism 320 is provided in the purging position, and includes a cap 321, a suction pump 322, and a purge tank 325. The cap 321 includes a seal member 323. Then, the cap 321 is lifted up and seals the print head 301. In this condition, the suction pump 322 sucks up and removes air bubbles along with ink from the nozzles 302.
However, it is difficult to remove air bubbles from the common ink chamber 307, a connecting portion between the ristrictor 306 and the common ink chamber 307, a connection portion between the common ink chamber 307 and the supply path 308, and the supply path 308 even in the above-described purging operation. In order to remove these air bubbles, it is conceivable to use a suction pump with greater power. It is also conceivable to drive the suction pump 322 for a increased time duration. However such operations increase the size of the ink jet print device and also decrease the printing speed. Also, the amount of ink consumed during the purging operation increases, which is uneconomical.
It is an objective of the present invention to overcome the above problems, and to provide an ink supply method for reliably removing air bubbles remaining in print head without wasting ink, and also to provide an ink jet print device that performs the ink supply method.
In order to achieve the above and other objectives, there is provided an ink jet print device including a head, a sub ink tank, a main ink tank, a first switching valve, and a second switching valve. The head is formed with a plurality of nozzles and a common ink chamber in fluid communication with each of the nozzles. Each nozzle is formed with an orifice through which an ink droplet is ejected. The common ink chamber has a first end and a second end. The sub ink tank stores ink and supplies the ink to the nozzles. The main ink tank stores ink and supplies the ink to the sub ink tank. The first switching valve is provided to the first end of the common ink chamber, and is selectively switched to an opening condition where the common ink chamber is in fluid connection with the sub ink tank and a closed condition where the common ink chamber is in fluid disconnection from the sub ink tank. The second switching valve is provided to the second end of the common ink chamber. The second switching valve is selectively switched to a first condition where the common ink chamber is in fluid connection with the sub ink tank and fluid disconnection from the main ink tank and a second condition where the common ink chamber is in fluid connection with the main ink tank and fluid disconnection from the sub ink tank. The sub ink tank supplies the ink to the nozzles through the common ink chamber, and the main ink tank supplies the ink to the sub ink tank through the common ink chamber.
There is also provided an ink supply method including the steps of a) switching a first valve to fluidly connect to a first side of a common ink chamber to a sub ink tank, b) switching a second valve to fluidly connect a main ink tank to a second side of the common ink chamber and to fluidly disconnect the sub ink tank from the second side of the common ink chamber, and c) driving a pump provided between the main ink tank and the second side of the common ink chamber so as to provide ink from the main ink tank through the common ink chamber into the sub ink tank.